1. Field of the Invention
The present invention relates to a complex type sigma-delta analog to digital conversion unit and a receiver having the same.
2. Description of the Related Art
FIG. 7 is a view showing the configuration of a radio receiver having a real type sigma-delta analog to digital conversion unit described in the following Patent Document 1. A mixer 701a mixes an input signal with a 0° phase shift signal to generate an I signal. A mixer 701b mixes an input signal with a 90° phase shift signal to generate a Q signal. The I signal and Q signal are signals that are orthogonal to each other. A real type sigma-delta analog to digital conversion unit 700a, which includes a subtractor 711, a loop filter 712, an analog to digital (A/D) converter 713, and a digital to analog (D/A) converter 714, converts the I signal into a digital format from an analog format. A real type sigma-delta analog to digital conversion unit 700b, which includes a subtractor 711, a loop filter 712, an analog to digital (A/D) converter 713, and a digital to analog (D/A) converter 714 like the analog to digital conversion device 700a, converts the Q signal into a digital format from an analog format.
FIG. 8 is a view showing the configuration of a radio receiver having a complex type sigma-delta analog to digital conversion unit described in the following Patent Document 2. A mixer 801a mixes an input signal with a 0° phase shift signal to generate an I signal. A mixer 801b mixes an input signal with a 90° phase shift signal to generate a Q signal. The I signal and Q signal are signals that are orthogonal to each other. A complex type sigma-delta analog to digital conversion unit 800 includes subtractors 811a and 811b, loop filters 812a and 812b, analog to digital (A/D) converters 813a and 813b, and digital to analog (D/A) converters 814a and 814b. The output signal of the loop filter 812a is outputted to the loop filter 812b with its positive-negative sign maintained and, and the output signal of the loop filter 812b is outputted to the loop filter 812a with its positive-negative sign inverted.
[Patent Document 1] Translated National Publication of Patent Application No. 2003-527795
[Patent Document 2] U.S. Pat. No. 5,764,171
FIG. 9 is a view showing a quantization noise spectrum of the real type sigma-delta analog to digital conversion unit of FIG. 7. When an input signal is converted into a digital format from an analog format, a quantization noise due to the quantization will be generated. The quantization noise spectrum is symmetrical with respect to the axis of which the frequency is 0 Hz. The frequency range RN of the input signal includes positive and negative low-frequency ranges having a predetermined quantization noise or less. The positive frequency range of the input frequency range RN is a positive signal bandwidth CW.
FIG. 10 is a view showing a quantization noise spectrum of the complex type sigma-delta analog to digital conversion unit of FIG. 8. The difference between the real type and complex type sigma-delta analog to digital conversion units is whether the loop filter has a complex pole or not. In the real type of FIG. 9 the quantization noise spectrum becomes symmetrical with respect to the axis of the frequency is 0 Hz, while in the complex type of FIG. 10 the quantization noise spectrum shifted in the positive direction of the frequency axis can be obtained. In realizing a reduction in the quantization noise in the signal bandwidth CW with the same filter order, the same filter cut-off frequency, the same sampling frequency, and the nearly same power consumption, the complex type of FIG. 10 can reduce it much more as compared with the real type of FIG. 9.
However, in the real type of FIG. 9 the quantization noise level in the negative input frequency range is nearly equal as compared with the one in the positive input frequency range, while in the complex type of FIG. 10 the quantization noise level in the negative input frequency range becomes extremely large as compared with the one in the positive input frequency range and thus the input frequency range RN is restricted to a positive range.